This invention relates to a null steering conformal cylindrical antenna system, and more particularly to such a system which prevents jamming of GPS signals.
The Global Positioning System (GPS) has become an important component of civilian and military navigation and locating systems. GPS signals, however, typically in the range of 1.5 GHz, are quite weak and easily jammed. Jamming signals are typically relatively very high power; usually they are the highest power incident signals and they come from specific changing directions.
Conventional implementations of GPS antennas on cylindrical vehicles or projectiles, e.g., munitions, typically include an array of circularly polarized elements whose outputs are combined to produce an omnidirectional gain pattern that can provide no rejection of jamming power and therefore does not enhance GPS performance.
Another technique selects certain circularly polarized elements known to point only in the direction of the GPS satellite signal source(s) and, in so doing, provides suppression of the jammer. This concept, however, is limited to applications in which the orientation of the munitions about its axis relative to the GPS satellites is known a priori. Such a technique is ineffective in gunfire applications where the round is spinning and its orientation is generally unknown during the GPS signal acquisition. A third technique attempts to implement a null steering system using circularly polarized elements which are large and, therefore, must be fewer in number and/or more closely spaced about the surface of the munition. Insufficient array elements, or a close spacing contributes to undesirable mutual coupling, and is detrimental to the formation of good jammerrejecting nulls. Poorer performance in this latter method is further exacerbated by depolarization of signals received at oblique angles. All of these effects are most pronounced on the small surface areas which characterize 105 mm, 5xe2x80x3 and 155 mm caliber projectiles. It is within this class of munitions that the need for jammer rejection is greatest and is most difficult to obtain.
It is therefore an object of this invention to provide an improved null steering antijamming antenna system.
It is a further object of this invention to provide such a null steering antijamming antenna system which is conformal and simple.
It is a further object of this invention to provide such a null steering antijamming antenna system which is effective with low cost microstrip antennas.
It is a further object of this invention to provide such a null steering antijamming antenna system which is effective with cylindrical bodies including projectiles and munitions.
It is a further object of this invention to provide such a null steering antijamming antenna system which is effective with rotating bodies.
It is a further object of this invention to provide such a null steering antijamning antenna system which has a more uniform coverage pattern and reduces mutual coupling interference.
It is a further object of this invention to provide such a null steering antijamming antenna system which decreases a jamming signal relative to the GPS signal.
The invention results from the realization that a truly effective antijam null steering antenna system capable of improving GPS signal to noise performance relative to jamming signals can be conformed to cylindrical bodies, even small ones like projectiles and munitions, by using a cylindrical array of linearly polarized individually weighted antenna elements and associated weighting circuits with a constrained power minimization circuit such as an Applebaum circuit or least mean squares circuit to always maintain reduced sensitivity in the direction of the maximum incident power or jamming signal. This realization results from the recognition that, while using the linearly polarized antenna the system reduces the GPS signal reception by a factor of two compared to circularly polarized antennas, the noise rejection obtainable can reduce the jamming signal by a factor of a thousand and thus greatly enhance overall GPS signal recovery.
This invention features an antijam null steering conformal cylindrical antenna system including a cylindrical array of linearly polarized individually weighted antenna elements. There are a plurality of weighting circuits one associated with each of the elements and means, responsive to the power output of the weighting circuits, for decreasing the sensitivity in the direction of the maximum incident power.
In a preferred embodiment the cylindrical array may be a right circular cylinder array. The elements may be approximately one half wavelength long, one quarter wavelength wide, and spaced on one half wavelength centers. There may be at least four such elements. The weighting circuit may include a gain circuit. The antenna system may be a GPS receiver antenna system having a frequency of approximately 1.5 GHz. The means for decreasing sensitivity may include various circuits for constrained minimization of power at the output of the subject antenna system, e.g., a least mean square circuit or an Applebaum circuit.